Guard against params[:form] being nil

Apparently this happens on 1.8.7.
http://travis-ci.org/#!/jnicklas/capybara/jobs/2181888/L69

This is perhaps Sinatra acting different under 1.8.7. I'm not sure what
exactly is going wrong, but it doesn't seem to be a bug in Capybara
(just the test suite).

lib/capybara/spec/test_app.rb

@@ -107,7 +107,7 @@ class TestApp < Sinatra::Base
   end
 
   post '/upload_empty' do
-    if params[:form][:file].nil?
+    if params[:form].nil? || params[:form][:file].nil?
       'Successfully ignored empty file field.'
     else
       'Something went wrong.'